b43: work around a locking issue in ->set_tim()

ops->set_tim() must be atomic, so b43 trying to acquire a mutex leads
to a kernel crash. This patch trades an easy to trigger crash in AP
mode for an unlikely race condition. According to Michael, the real
fix would be to allow set_tim() to sleep, since b43 is not the only
driver that needs to sleep in all callbacks.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>